A federal tribunal has rebuked the government for hiding documents in a challenge over funding for First Nations children.

The Canadian Human Rights Tribunal says the federal government knew about 50,000 documents "prejudicial to its case and highly relevant" in 2012, but didn't disclose them until they were revealed this year in a request under federal Access to Information laws. That evidence makes up two-thirds of the records to be handed over in the challenge.

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Cindy Blackstock has brought a challenge against the federal government over the level of funding for First Nations children's services. (CBC)

Cindy Blackstock, the woman who mounted the challenge on behalf of the First Nations Child and Family Caring Society of Canada, only found out the documents existed after filing a request under access laws.

The tribunal says in its decision that the government knew it wouldn't be able to finish disclosing relevant documents in time to meet its deadline, but didn't mention the fact during at least two meetings about the hearing this spring.

"The Respondent [the government] withheld this information from the parties and the tribunal. Only following the Caring Society’s letter regarding the [access] request, in a letter dated May 7, 2013, shortly before the third week of the hearing was scheduled to commence, did the Respondent inform the parties and the tribunal of the existence of 50,000 additional outstanding disclosure documents," the tribunal says in its decision.

The tribunal has also ordered the government to do a rolling disclosure every three weeks to prevent a last-minute avalanche of documents on the Aug. 31 deadline.

The rebuke comes out of a decision handed down in June. The reasons for the decision were only released Wednesday to speed up the ruling. The tribunal noted that the proceeding, which started in 2007, has already been "marked by a number of significant delays."